Workflow for Optimizing Horizontal Well Completions in Thinly Bedded Sandstone Reservoirs

Abstract

Abstract Recently appraised reservoirs in Alaska have hydrocarbons stored in highly laminated reservoirs with sandstone pays interbedded with tight mudstone and siltstone non-pays. Vertical flow barriers exist such that long horizontal wells with multi-stage longitudinal hydraulic fractures prove to be an effective way to connect the separated pay intervals and to increase the waterflooding sweep efficiency. This study presents a workflow to optimize the lower completion and hydraulic fracturing design for developing reservoirs with very limited vertical permeability. Laboratory tests were performed to characterize the rock mechanical properties. The test results are used to predict formation failure and determine whether the lateral needs to be cemented to isolate weak rock. Fracture conductivity laboratory tests were also performed to determine the resultant conductivity for different proppant types and concentrations. A series of well inflow models were then constructed to investigate the completion design parameters such as the optimum frac sleeve spacing and required proppant concentration. Fracture models were built to determine the fracturing design that can deliver these parameters for the optimal reservoir development. Laboratory geomechanics testing of thinly bedded laminated rock proved challenging and best practices to determine Poisson's ratio and UCS are presented. Formation failure modeling shows that a cemented completion is preferred when the wellbore is widely exposed to the weak, water-sensitive layers which are predicted to fail during early flowback period. In un-cemented liner completions, the weak intervals need to be mechanically isolated so that the failed rocks do not impede flow through the annulus between the openhole and production liner, sleeve ports or tubing. Fracture conductivity testing showed high proppant concentrations are required to achieve the conductivity targets due to a significant loss of fracture conductivity. Under idealistic conditions, un-cemented completions result in higher well productivity than cemented completions if the longitudinal fractures are fully aligned with the wellbore and the openhole annulus stays unplugged during the well life. However, well productivity for un-cemented liner completion decreases drastically when the wellbore misaligns with the fracture or formation failure occurs near the sleeve ports. Well productivity with cemented completions can be significantly improved by minimizing the near-field pressure drop through higher fracture conductivity and tighter frac sleeve spacing. This study presents an integrated workflow to design effective completions for reservoirs with very limited vertical permeability. This couples the inflow performance model with the fracture model results for horizontal wells with multi-stage longitudinal fractures to optimize critical completion and fracture design parameters.